How do the dumbbell- and bola-shaped amphiphiles assemble: vesicles with condensed hydrophobic domains or blackberry-type structures with porous surfaces?*

Dumbbell-shaped hybrid amphiphiles, with two large, rigid, charged hydrophilic heads and a hydrophobic flexible linker in between, can assemble into hollow, spherical supramolecular structures in polar solvents. The question is if they assemble into conventional vesicles with a condensed hydrophobic domain via hydrophobic interaction, or blackberry-type structures via counterion-mediated attraction with porous assembly surfaces. These two types of assemblies have different driving forces and mechanisms but similar apparent assembly structures; both are possible.

We have demonstrated that some dumbbell-shaped amphiphiles form regular vesicles, while another type of dumbbell hybrid, AC₆₀-AC₆₀, containing two carboxylic acid-functionalized C₆₀ (AC₆₀) polar heads linked by an organic tether, behave as macroions and assemble into blackberry-type structures. We distinguish between vesicles and blackberries by examining the size response of assemblies on solvent polarity and pH of aqueous solutions. In various solutions, the assembly size responds to environmental change with features for blackberry structures. The surface porosity of assemblies is also examined by all-atom simulations. Furthermore, a bolaform macromolecule, AC₆₀-PEG-AC₆₀, with a hydrophilic polyethylene glycol (PEG) chain as the linker, can also self-assemble and show similar pH responses as assemblies from AC₆₀-AC₆₀. Therefore, the nature of these assemblies is blackberry-type structures rather than vesicles.